1. Field of the Invention
The present invention relates to a single-mode single-polarization (SMSP) optical fiber and, more particularly to an SMSP optical fiber wherein high stress-induced birefringence is utilized to split the two polarizations of the fundamental mode and a cladding structure is included which attenuates one polarization by tunneling. The resultant fiber may, therefore, be utilized as a fiber polarizer.
2. Description of the Prior Art
Optical waveguides capable of transmitting power with only one direction of polarization are desirable in many different applications, such as fiber sensors, in-line fiber devices, Raman lasers, and the like. However, an ordinary axially symmetrical single-mode fiber is a "two-mode" fiber because two orthogonally polarized HE.sub..parallel. modes can be propagated in it. This fact results in the instability of the polarization state of the propagated mode when geometrical or dielectric perturbations exist in the fiber. To minimize these adverse effects, polarization-preserving fibers, as distinguished from single polarization fibers, have been developed, where these fibers may be categorized into three basic types; (1) elliptical-core fibers, (2) stress-induced birefringent fibers, and (3) side-pit fibers.
One example of a prior art elliptical core fiber is found in U.S. Pat. No. 4,106,847 issued to J. A. Arnaud on Aug. 15, 1978, where it is shown that an elliptical core fiber, as opposed to a circular core fiber, may be more efficiently coupled to a light emitting source having an elongated source area, such as in a heterojunction laser or an edge-emitting light emitting diode.
U.S. Pat. No. 4,274,854 issued to W. Pleibel et al on June 23, 1981 discloses a prior art method of fabricating a polarization-preserving optical fiber which produces a stress-induced birefringence therein. The method includes depositing the cladding layers and the core layer within a specially-shaped substrate tube, collapsing the resultant product into a preform and drawing a fiber therefrom. The difference in thermal expansion among the various layers, combined with their inherent non-circularity, provide the stress-induced birefringence and hence the polarization-preserving properties of the resultant optical fiber.
All of the above-cited prior art arrangements, however, relate to polarization-preserving fibers, where such fibers are not capable of suppressing, or eliminating, an undesired polarization of the transmitted mode. A scheme which was intended to suppress the undesired polarization is the side-pit fiber arrangement described in the article "Single-Polarization Single-Mode Optical Fibre With Refractive-Index Pits on Both Sides of the Core" by T. Okoshi et al appearing in Electronics Letters, Vol. 16, No. 18, Aug. 28, 1980 at pp. 712-713. An axially nonsymmetrical index distribution featuring two pits is proposed, one pit on either side of the core. In this structure, the two orthogonally polarized HE.sub..parallel. modes have cutoff wavelengths which differ so that in a certain narrow frequency range one mode is attenuated while the other is transmitted. At higher frequencies, both polarizations propagate and the fiber will preserve linear polarization along both principal axes. The effective birefringence is geometrical in nature and is controlled by the index difference between the core, the cladding, and the side pits. For a relative index difference (.DELTA.n/n) of 1% the predicted single-polarization bandwidth is about 2.5%. It was suggested by Okoshi et al that for a practical single-polarization fiber a relative index difference of approximately 2% would be required.
The side pit structure was investigated experimentally in an article entitled "Single Mode Fibres with Asymmetrical Refractive Index Pits on Both Sides of Core" by T. Hosaka et al appearing in Electronics Letters, Vol. 17, No. 5, March, 1981, at pp. 191-193. Here it is verified that side pit fibers preserve linear polarization along the fiber's two principal axes. No single-polarization (in the sense of action as a fiber polarizer) was reported. It was later learned that much of the birefringence of the fiber is stress-induced and modifications were introduced to produce high-birefringence, low-loss polarization preserving fibers as described in the article "Low-Loss Single Polarization Fibres with Asymmetrical Strain Birefringence" by T. Hosaka et al appearing in Electronics Letters, Vol. 17, No. 15, July 23, 1981 at pp. 530-531. An alternative form of side-pit fiber is described in the article "Side-Tunnel Fibre: An Approach to Polarisation-Maintaining Optical Waveguiding Scheme" by T. Okoshi et al appearing in Electronics Letters, Vol. 18, No. 19, Sept. 26, 1982 at pp. 824-826, where the side pits are replaced by hollow tunnels. Such fibers are predicted to have bandwidths of 7% for a 1% relative index difference. In practice, however, true single polarization behavior has not been observed.
Thus, there remains a need for a fiber polarizer capable of completely suppressing one of the fundamental mode polarizations, is capable of operating over a large bandwidth, is compatible with typical fibers and lends itself to readily available production techniques.